1. Technical Field of the Invention
The present invention relates generally to radio frequency (RF) transmitters and, more particularly, to providing accurate and efficient control over power being transmitted from a third generation 3G mobile phone.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems include national and/or international cellular telephone systems, the Internet, and point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards or protocols. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), Enhanced Data rates for GSM Evolution (EDGE), General Packet Radio Service (GPRS), and extensions and/or variations thereof.
Mobile communication has changed the way people communicate and mobile phones have been transformed from a luxury item to an essential part of every day life. The use of mobile phones today is generally dictated by social situations, rather than being hampered by location or technology. While voice connections fulfill the basic need to communicate, today's mobile phones now incorporate technology to perform high speed data transfer, in order to access the Internet, download application programs (“apps”), games, audio, video, including movies and television programs. Third generation (3G) cellular networks have been specifically designed to fulfill much of these future demands for mobile phones. In this regard, universal mobile telecommunications system (UMTS) using wideband CDMA (WCDMA) has been developed as the 3G successor to GSM, GPRS and EDGE.
In operating a 3G mobile phone, the phone's transmit power is controlled by a 3G network to form a close-loop controlling scheme to utilize the capacity of spread spectrum and to ensure similar quality of communication connections to multiple users in the same cell network. Power control is extremely important in a WCDMA/UMTS system, posing multiple technical challenges for both base station (BS) and mobile phone or mobile station (MS) designs. The goal of transmit power control in a 3G network is to fully utilize the capacity of spread spectrum to allow the BS receiving the transmitted power from all MSs in the same cell sharing the same channel to have approximately equal quality of service.
Transmit power control for a 3G mobile phone entails open loop power control, outer loop power control and inner loop power control. The open loop power control is executed between a radio network controller (RNC) affiliated with a base station (BS) and a mobile phone, which is also referred to as a mobile station (MS). The open loop power control simply establishes a rough initial power setting for the MS.
The outer loop power control, which may also be referred to as slow close-loop power control, is executed between the RNC and MS at an approximate rate of 10-100 Hz. The RNC looks for a target block error rate (BLER) or a bit error rate (BER) that is specified for comparison with the estimated BLER or BER from the MS. The RNC then determines a target SIR (Signal Interference Ratio) for the inner loop power control based on the estimated BLER or BER.
The inner loop power control (ILPC), which may also be referred to as fast close-loop power control, is executed between the BS and the MS at an approximate rate of 1500 Hz to meet the target SIR determined by the BLER or BER target set by the outer loop power control. If the received SIR is lower than the target SIR threshold, the BS will issue a transmit power control (TPC) command to the MS to increase the transmit power. Alternatively, if the received SIR is higher than the target SIR threshold, the BS will issue a TPC command to the MS to decrease transmit power. If the received SIR is within a target SIR threshold range, the BS will issue a TPC command to the MS to maintain the same transmit power.
For the ILPC executed between the BS and the MS, two major technical difficulties have to be overcome to achieve accurate and efficient transmit power control for mobile phones. First, the ILPC requires a WCDMA/UMTS phone to be able to set up a transmit power level, at an accuracy of ±0.5 dB, controlled by a TPC command in an upcoming WCDMA/UMTS time slot of 0.67 mS (or at a 1500 Hz/s). The required ILPC accuracy and related speed are difficult to meet for the following reasons. Transmit power level in a mobile phone is usually a function of multiple variables, such as channel frequency ripple, temperature and nonlinearity. In providing factory calibration, the transmit power calibration performed at room temperature cannot cover all possible variations introduced during phone operation. It is very difficult to set an accurate digital-to-analog (DAC) value at once in the transmitter for a precise transmit power level and to adjust the DAC value within the allotted time period, based on transmit power level feedback, to achieve the final required transmit power level in response to the issued TPC command. Moreover, if a predetermined offset adjustment is used to compensate for a ripple based on the channel frequency, temperature variation and nonlinearity in the transmit chain, the offset adjustment is only accurate as the calibration scheme that is used to obtain the offset values. Due to the complicated relationship between the transmit power level and the variables encountered in the transmit path, it is difficult to obtain accurate offset values for the required transmit power level.
The second major technical difficulty is due to the medium of transmission, which may also be referred to as the radiated transmission. Since a MS communicates with the BS through an air interface, the MS experiences more power control fluctuation, and accuracy and reliability may be diminished for the following reasons. The fading variations caused by the phone's mobility and multipath air interface create transmit power control spikes and fluctuations. Due to antenna diversity, variations caused by multipath diversity, receiver diversity and transmit antenna diversity also directly affect the transmit power level. The delays in the power control loop, both air-interface delay and phone's circuitry delay, as well as inaccurate SIR estimates, may also increase the inaccuracy and unreliability of the transmit power. Moreover, the transmit power spikes and fluctuations created from the above factors may directly introduce interference to other users in the same cell network, and make the BS power control fluctuate even more.
To address some of these noted problems, many 3G power control efforts are focused on operations at the BS, such as BS based algorithm implementations. However, the efforts on mobile phone designs are mainly focused on building fast response close-loop transmit power control, which may or may not achieve the ILPC requirements, partly due to the technical complications and difficulties noted above.
Therefore, a need exists to provide a transmit power compensation scheme at the phone end of the communication link to accurately and efficiently generate transmit power in response to a TPC command issued by the BS.